Clear Variable in JavaScript

function loadHistory(itemlist) {
    if (itemlist != null) {

        var gridView = jQuery("#GridView1");
        jQuery(".GridviewScrollItem").remove();
        jQuery(".GridviewScrollHeader").remove();
        //gridView.append("<tr class\"GridviewScrollHeader\"></tr>");

		if (itemlist.length > 0){
//		    jQuery.each(returnObject, function (key, value) {
		        //this is where I want the clear function to go.
                        //so that every time I run the code, "gridViewItem" and "gridViewHeader"
                        //the value will be like I pressed refresh. Hoping there is a textbox.clear function
                        var gridViewItem = "";
		        var gridViewHeader = "";
		        
		        gridViewHeader += "<tr><th class=\"headcol\">Time</th><th>Alarm</th>";
		        gridViewHeader += " <th>Power</th><th>Defrost</th>";
		        gridViewHeader += " <th>Setp</th><th>Sup</th>";
		        
		        for (var i = 0; i < itemlist.length; i++) {
		            //var targetObject = itemlist[i];
		            var m1 = moment(itemlist[i].ReceptionTime);
		            gridViewItem = gridViewItem + "<tr style=\"visibility:visible;\">";
                    //Time
		            if ((m1 != null) && (itemlist[i].ReceptionTime != null))
		                gridViewItem = gridViewItem + " <td align=\"right\" class=\"headcol\">" + moment(itemlist[i].ReceptionTime).format("MM/DD/YYYY HH:mm") + "</td>";
		            else gridViewItem = gridViewItem + " <td  align=\"right\" class=\"headcol\"></td>";

		            //Alarms
		            if (itemlist[i].Alarms.length != 0)
		                gridViewItem = gridViewItem + " <td align=\"right\">" + itemlist[i].Alarms.length + "</td>";
		            else gridViewItem = gridViewItem + " <td align=\"right\"></td>";

		            //PowerState
		            if (itemlist[i].PowerState != null) {
		                //gridViewItem = gridViewItem + " <td align=\"right\">" + itemlist[i].PowerState + "</td>";
                        if (itemlist[i].PowerState == "0")
                            gridViewItem = gridViewItem + " <td align=\"right\">ON</td>";
                        else
                            gridViewItem = gridViewItem + " <td align=\"right\">OFF</td>"; 
		            }
		            else gridViewItem = gridViewItem + " <td align=\"right\"></td>";

		            //Defrost
		            if (itemlist[i].Defrosting != null){
		                gridViewItem = gridViewItem + " <td align=\"right\">" + itemlist[i].Defrosting + "</td>";
//                        if (itemlist[i].Defrosting == false)
//		                    gridViewItem = gridViewItem + " <td align=\"right\">YES</td>";
//                        else
//                            gridViewItem = gridViewItem + " <td align=\"right\">NO</td>";
                    }
		            else gridViewItem = gridViewItem + " <td align=\"right\"></td>";

		            //SetPoint
		            if (itemlist[i].Setpoint != null)
		                gridViewItem = gridViewItem + " <td align=\"right\">" + itemlist[i].Setpoint.toFixed(1) + "</td>";
		            else gridViewItem = gridViewItem + " <td align=\"right\"></td>";

		            //Supply
		            if (itemlist[i].Supply != null)
		                gridViewItem = gridViewItem + " <td align=\"right\">" + itemlist[i].Supply.toFixed(1) + "</td>";
		            else gridViewItem = gridViewItem + " <td align=\"right\"></td>";

                    gridViewItem = gridViewItem + "</tr>";
		        }

		        //jQuery(".GridviewScrollHeader").append(gridViewHeader);
                gridView.append(gridViewHeader + gridViewItem);
		        //gridviewScroll();//here is the problem! jQuery Freeze Columns.
		    //});
		}
	}
}